1. Field of the Invention
The present invention relates, in general, to microwave ovens. More specifically, the invention is directed to a microwave oven having a substantially continuously variable output power, even in a low power range.
2. Description of the Prior Art
Microwave ovens having a variable magnetron output power are known. They generally employ an inverter circuit which supplies the magnetron with an A.C. voltage obtained by inverting rectified commercial A.C. power. An inverter circuit can continuously vary its output by changing the ON-OFF period of a switching element thereof. Therefore, it would seem most suitable to use an inverter circuit to control the magnetron for different kinds of food or cooking.
However, there is a problem in using an inverter circuit as described above. It is difficult to control magnetron output in a "low" output power range. Typically, a power transistor switching element is controlled on and off by a high frequency. The collector current-waveform of the power transistor of the inverter circuit fluctuates as magnetron output approaches 0%. A breakdown of the power transistor may ultimately occur.
To avoid such breakdown the magnetron is arranged to provide, whenever it operates, at least a minimum power output, such as, for example, 200 watts. No power control below that minimum output is carried out. As a practical matter therefore, it is not always possible to obtain a suitable level of magnetron output power for a given cooking requirement.